1. Field of the Invention
The present invention relates to a process for etching a film of boron nitride with high selectivity to a layer of silicon dioxide or silicon nitride.
2. Description of the Prior Art
In recent years, silicon nitride films have been increasingly used as interlevel dielectrics in the fabrication of ULSI devices. Silicon nitride films have found favor because they provide various advantages, such as good insulation characteristics, low deposition temperature and high passivation effect against moisture and alkali metal ions, as well as satisfactory conformal step coverage, crack resistance, etc. However, the dielectric constant of silicon nitride is relatively high, and this causes relatively large parasitic capacitance and relatively long propagation delay times between devices. In order to reduce the wiring capacitance, a dielectric material having a low dielectric constant is required; this material should also provide the other advantages provided by silicon nitride films.
One approach to solving the above problem is to use silicon dioxide as an interlevel dielectric. While silicon nitride has a dielectric constant of 7, silicon dioxide has a dielectric constant of 4. Recently, however, it has also been demonstrated that boron nitride films, that are boron-rich and prepared by thermal CVD (1000.degree. C.), have a low dielectric constant of less than 4, with satisfactory insulation and thermal stability characteristics. For a review of various techniques for preparation of boron nitride films, see Arya, et al., "Preparation, Properties And Applications Of Boron Nitride Thin Films," Thin Solid Films, 157 (1988), pp. 267-282.
Boron nitride films having a low dielectric constant have also been prepared by introduction of a small amount of carbon or silicon. For example, in Yamada, et al., "Improvements Of Stress Controllability And Radiation Resistance By Adding Carbon To Boron-Nitride", J. Electrochem. Soc., Vol. 137, No. 7 (Jul. 1990), pp. 2242-2246, a conventional plasma-enhanced CVD system, using a gas mixture of diborane, ammonia and methane, was employed to deposit carbon-containing boron nitride films (400.degree.-500.degree. C.). In Maeda, et al., "Low Dielectric Constant Amorphous SiBN Ternary Films Prepared By Plasma-Enhanced Deposition", Jap. J. of Applied Physics, Vol. 26, No. 5 (May 1987), pp. 660-665, silicon-containing boron nitride films were deposited by conventional plasma-enhanced CVD using a diborane, ammonia and silane gas mixture.
In the fabrication of ULSI devices, interlevel boron nitride films are etched to form contact vias. A number of techniques are disclosed in the art for etching boron nitride films. For example, U.S. Pat. No. 4,226,665, issued Oct. 7, 1980 to Mogab, describes the use of a plasma formed from CF.sub.4, with a small amount of oxygen (8%), to etch boron nitride and silicon nitride films. Also Maeda, et al., "A Low-Permittivity Interconnection
Using An SiBN Interlayer", IEEE Transactions On Electron Devices, Vol. 36, No. 9 (Sep. 1989), pp. 1610-1614, show the use of oxygen or a CF.sub.4 /H.sub.2 mixture to etch a SiBN layer, overlying a metallization pattern (Al-Si).
During etching of the films, it is also desirable that the overlying mask be replicated in the boron nitride without dimensional loss, i.e. that the etch be anisotropic. Further, it is important that there be minimal etching of the mask, typically silicon nitride or silicon dioxide, i.e. that the etch be highly selective to the overlying mask. Selectivity to silicon nitride and silicon dioxide is also required in that the boron nitride film frequently overlies a layer of one of these materials.
Processes which selectively etch to overlying or underlying layers of silicon dioxide have been developed, for use where silicon nitride is the interlevel dielectric material. See, for example, U.S. Pat. No. 4,793,897, issued Dec. 27, 1988 to Dunfield, et al., which discloses a process for etching silicon nitride selectively to underlying silicon dioxide, by employing a gaseous mixture of a fluorine-containing gas, e.g. SiF.sub.4, and oxygen; the mixture contains a predominant amount of the fluorine-containing component. Such a process is also disclosed in U.S. Pat. No. 4,303,467, issued Dec. 1, 1981 to Scornavacca, et al., which discloses a plasma treatment with SiF.sub.4.
Other processes for etching silicon nitride or silicon dioxide are disclosed in U.S. Pat. No. 4,374,698, issued Feb. 22, 1983 to Sanders, et al. (etching of silicon nitride or silicon dioxide, by employing a mixture of CF.sub.4 and small amounts of oxygen and a halogen-containing compound, e.g. CF.sub.2 Cl.sub.2); and U.S. Pat. No. 4,814,041, issued Mar. 21, 1989 to Auda (etching of silicon dioxide with a fluorine-containing gas, e.g. CF.sub.4, and a small amount of oxygen).
Thus, there is a need in the art for a process for etching a film of boron nitride with high selectivity to a layer of silicon dioxide or silicon nitride, and which achieves anisotropic etch profiles, with reasonably high etch rates.